1. Field of the Invention
This invention relates to a cryogenic vessel having an interior ullage vessel and, more specifically, to a cryogenic vessel having an interior ullage vessel that includes a venturi assembly with a fill line passing therethrough or coupled thereto.
2. Background Information
Cryogenic liquids, such as liquid natural gas (LNG), nitrogen, oxygen, CO2, hydrogen and the like, are substances that normally exist as gasses, but are liquids at cold temperatures. Special vessels and systems must be used to store and transfer cryogenic liquids because of difficulty in maintaining the extremely cold temperatures. Such vessels typically include a double walled vessel having a vacuum in the annulus or plenum. While the vacuum provides an effective insulation, the insulation is not perfect and, as such heat penetrates the vessel. When heat is added to the cryogenic liquid, a portion of the liquid returns to the gaseous state. The gas within the vessel increases the internal pressure. Eventually, to prevent over pressurization of the vessel, the gas must be vented. It is desirable to prevent, or at least delay, the venting of the gas.
One means of delaying the over pressurization of the vessel was to not completely fill the vessel with the cryogenic liquid. That is, vessels were filled by spraying the cryogenic liquid into the top of the tank via a nozzle. This spray would condense the gas in the tank and collapse the pressure head therein. When the pressure head collapsed, the pressure in the tank was substantially reduced so that the cryogenic liquid could flow into the tank. The cryogenic liquid flowed into the tank until the cryogenic liquid submerged the nozzle. Once the nozzle was submerged, the pressure in the tank would gradually increase because the incoming cryogenic liquid was no longer contacting the gas within the vessel and, therefore, unable to collapse the pressure head. A pressure gage monitored the fill line and the filling procedure was automatically shut-off when the pressure in the tank reached a predetermined value or at a predetermined flow rate reduction. At this point, the vessel was not completely full and an ullage space existed above the level of the cryogenic liquid. The ullage space accommodated any cryogenic liquid that was subsequently vaporized due to heat penetration. Because the newly vaporized gas had a space to fill, the hold time of the tank was increased and venting was delayed.
Because monitoring pressure in the fill line to determine when the tank was filled to the proper point does not always result in the vessel being filled to the same level, this system was improved by the addition of an ullage space vessel. The ullage space vessel was essentially another vessel having a small opening therein, and which was disposed within the cryogenic vessel. During the fill procedure, the small opening in the ullage space vessel prevented the ullage space vessel from being filled as rapidly as the cryogenic vessel. That is, a small amount of cryogenic fluid would seep into the ullage space vessel at a slow rate. This addition of cryogenic liquid to the ullage space vessel, as well as the fact the ullage space vessel was surrounded by the cryogenic liquid, caused the pressure head in the ullage space vessel to collapse. When the cryogenic vessel was filled completely, the cryogenic liquid would back up the fill line causing a very noticeable increase in pressure and a decrease in the fill rate. At this point the fill procedure stopped. Thus, the cryogenic vessel was completely filled while the ullage space vessel was, essentially, empty. The cryogenic liquid would continue to seep into the ullage space tank until the fill levels were essentially even. As the liquid seeped into the ullage space vessel, an ullage space was created above the cryogenic liquid. This ullage space, as well as any vapor space in the ullage space vessel, provided a space for any vaporizing liquid to expand.
This system, however, also has disadvantages. For example, when a vessel being refilled is not completely empty, the cryogenic fluid in the ullage space vessel is at the same level as the cryogenic liquid in the storage space. Thus, the vapor space of the vessel is reduced. This limits the size of the final ullage space and may require subsequent venting of gas.
There is, therefore, a need for a cryogenic vessel with an ullage vessel structured to drain any residual liquid while resisting filling with a liquid during the filling procedure.
There is a further need for a cryogenic vessel with an ullage vessel having a fill line with a venturi assembly thereon structured to draw liquid from the ullage vessel during the filling procedure.